Semiconductor epitaxy bordering isolation structure

ABSTRACT

A method includes providing a semiconductor structure having an active region and an isolation structure adjacent to the active region, the active region having source and drain regions sandwiching a channel region for a transistor, the semiconductor structure further having a gate structure over the channel region. The method further includes etching a trench in one of the source and drain regions, wherein the trench exposes a portion of a sidewall of the isolation structure, epitaxially growing a first semiconductor layer in the trench, epitaxially growing a second semiconductor layer over the first semiconductor layer, changing a crystalline facet orientation of a portion of a top surface of the second semiconductor layer by an etching process, and epitaxially growing a third semiconductor layer over the second semiconductor layer after the changing of the crystalline facet orientation.

PRIORITY

The present application is a divisional application of U.S. patent application Ser. No. 15/475,826, filed on Mar. 31, 2017, now U.S. Pat. No. 10,147,609 issued Dec. 4, 2018, titled “Semiconductor Epitaxy Bordering Isolation Structure,” which claims the benefits of U.S. Provisional Application Ser. No. 62/434,966, filed on Dec. 15, 2016, titled “Semiconductor Epitaxy Bordering Isolation Structure,” the disclosure of each of which is incorporated herein in its entirety.

BACKGROUND

As semiconductor devices are scaled down progressively, strained source/drain (S/D) features (e.g., stressor regions) have been implemented using epitaxially grown semiconductor materials to enhance charge carrier mobility and improve device performance. For example, forming a metal-oxide-semiconductor field effect transistor (MOSFET) with stressor regions may epitaxially grow silicon (Si) to form raised S/D features for n-type devices, and epitaxially grow silicon germanium (SiGe) to form raised S/D features for p-type devices. Various techniques directed at shapes, configurations, and materials of these S/D features have been implemented to further improve transistor device performance. However, existing approaches in raised S/D formation have not been entirely satisfactory.

For example, forming raised S/D regions at an active region next to an isolation region (or structure) has been problematic. For example, trenches for growing epitaxial features at the boundary of the two regions may not have an ideal shape. Also these trenches are only partially surrounded by semiconductor material(s). As a result, epitaxial features grown from these trenches might be thinner than those grown completely within the active region. Consequently, when contact features are formed above these epitaxial features, contact landing might be slanted and contact resistance might be high. Improvements in these areas are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 shows a flow chart of a method of forming a semiconductor device, according to various aspects of the present disclosure.

FIGS. 2, 3, 4, 5, 6, 7, 8, and 9 illustrate cross sectional views of forming a target semiconductor device according to the method of FIG. 1, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The present disclosure in various embodiments is generally related to semiconductor devices and methods of forming the same. In particular, the present disclosure is related to forming raised epitaxial features in source and drain (S/D) regions of field effect transistors (FETs). According to an embodiment, some of the raised epitaxial features are formed adjacent to (or bordering) isolation structures, and include at least three layers of semiconductor materials. A first layer of the semiconductor material (e.g., silicon germanium) is epitaxially grown out of a trench partially surrounded by a semiconductor material (e.g., silicon). A second layer of the semiconductor material (e.g. silicon) is epitaxially grown over the first layer, and is then etched to change a crystalline facet orientation of at least a portion of its top surface. A third layer of the semiconductor material (e.g. silicon) is epitaxially grown over the second layer, wherein the changed crystalline facet of the second layer facilitates a vertical growth of the third layer of the semiconductor material. Advantageously, the third layer of the semiconductor material attains a desirable film thickness and facet for S/D contact landing. This and other embodiments of the present disclosure are further described by referring to FIGS. 1-9.

FIG. 1 illustrates a flow chart of a method 100 for forming semiconductor devices according to the present disclosure. The method 100 is an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 100, and some operations described can be replaced, eliminated, or relocated for additional embodiments of the method. The method 100 is described below in conjunction with FIGS. 2-9, which illustrate cross-sectional views of a semiconductor device 200 during various fabrication steps according to an embodiment of the method 100. The device 200 may be an intermediate device fabricated during processing of an integrated circuit (IC), or a portion thereof, that may comprise static random access memory (SRAM) and/or logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as p-type FETs (PFETs), n-type FETs (NFETs), FinFETs, metal-oxide semiconductor field effect transistors (MOSFET), and complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof. Furthermore, the various features including transistors, gate stacks, active regions, isolation structures, and other features in various embodiments of the present disclosure are provided for simplification and ease of understanding and do not necessarily limit the embodiments to any types of devices, any number of devices, any number of regions, or any configuration of structures or regions.

Referring to FIG. 1, at operation 102, the method 100 provides a structure (or semiconductor structure) 200 that includes a semiconductor substrate with various active regions for forming transistors, gate structures over the active regions, and isolation structures adjacent to the active regions. An embodiment of the structure 200 is shown in FIG. 2.

Referring to FIG. 2, the structure 200 includes a substrate 202. The substrate 202 is a silicon substrate (e.g., comprising silicon in crystalline {110} faces) in the present embodiment. Alternatively, the substrate 202 may comprise another elementary semiconductor, such as germanium; a compound semiconductor such as silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In yet another alternative, the substrate 202 is a semiconductor on insulator (SOI).

The substrate 202 includes an active region 204 that is isolated from other active regions of the substrate 202 by isolation structures 212 a and 212 b. In the present embodiment, the active region 204 is a p-type field effect transistor (FET) region, such as an n-well in a p-type substrate, for forming PFET. In another embodiment, the active region 204 is an n-type FET region for forming NFET. In yet another embodiment, the active region 204 includes both p-type FET region(s) and n-type FET region(s) for forming CMOS devices. In the present embodiment the active region 204 includes various source and drain (S/D) regions 206 a, 206 b, and 206 c, and channel regions 208 a and 208 b that are sandwiched between a pair of S/D regions 206 a-c. The S/D regions 206 a-c may include lightly doped source/drain (LDD) features, and/or heavily doped source/drain (HDD) features. For example, the LDD and HDD features may be formed by a halo or lightly doped drain (LDD) implantation, source/drain implantation, source/drain activation, and/or other suitable processes. Particularly, the S/D region 206 a is adjacent to the isolation structure 212 a, the S/D region 206 c is adjacent to the isolation structure 212 b, and the S/D region 206 b is completely within the active region 204.

The isolation structures 212 a and 212 b are at least partially embedded in the substrate 202 and may be formed of silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-k dielectric material, and/or other suitable insulating material. The isolation structures 212 a-b may be shallow trench isolation (STI) features. In an embodiment, the isolation structures 212 a-b are STI features formed by etching trenches in the substrate 202, filling the trenches with one or more isolating materials, and planarizing the isolating materials with a chemical mechanical planarization (CMP) process. The isolation structures 212 a-b may be other types of isolation features such as field oxide and LOCal Oxidation of Silicon (LOCOS). The isolation structures 212 a-b may include a multi-layer structure, for example, having one or more liner layers.

The structure 200 further includes various gate structures 220 a, 220 b, and 220 c. In the present embodiment, the gate structures 220 b and 220 c are disposed over the active region 204, while the gate structure 220 a is disposed over the isolation structure 212 a. Particularly, the gate structures 220 b and 220 c are disposed over the channel regions 208 a and 208 b, respectively, for forming field effect transistors. In an embodiment, the gate structure 220 a functions as a local interconnect, such as for connecting the S/D 206 a to other parts of the device 200. The gate structure 220 a includes a gate dielectric layer 222 a, a gate electrode layer 224 a, an L-shaped spacer 226 a, and a sidewall spacer 228 a. The gate structure 220 b includes a gate dielectric layer 222 b, a gate electrode layer 224 b, an L-shaped spacer 226 b, and a sidewall spacer 228 b. The gate structure 220 c includes a gate dielectric layer 222 c, a gate electrode layer 224 c, an L-shaped spacer 226 c, and a sidewall spacer 228 c.

The gate dielectric layer 222 a-c may include silicon oxide layer (SiO₂) or a high-k dielectric layer such as hafnium oxide (HfO₂), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitable metal-oxides, or combinations thereof. The gate dielectric layer 222 a-c may be formed by ALD and/or other suitable methods.

The gate electrode layer 224 a-c includes polysilicon in an embodiment. Alternatively, the gate electrode layer 224 a-c includes a metal such as aluminum (Al), tungsten (W), cobalt (Co), copper (Cu), and/or other suitable materials. The gate electrode layer 224 a-c may be formed by CVD, PVD, plating, and/or other suitable processes.

The L-shaped spacer 226 a-c may include a dielectric material, such as silicon oxide, silicon oxynitride, other dielectric material, or combinations thereof. The sidewall spacer 228 a-c may include a dielectric material, such as silicon oxide, silicon nitride, silicon oxynitride, other dielectric material, or combinations thereof. The L-shaped spacer 226 a-c and the sidewall spacer 228 a-c may be formed by deposition (e.g., CVD) and etching techniques.

Each of the gate structures 220 a-c may further include an interfacial layer under the respective gate dielectric layer, one or more dielectric hard mask layers over the respective gate electrode layer, and/or a work function metal layer. For example, the interfacial layer may include a dielectric material such as silicon oxide layer (SiO₂) or silicon oxynitride (SiON), and may be formed by chemical oxidation, thermal oxidation, atomic layer deposition (ALD), CVD, and/or other suitable dielectric. For example, the hard mask layers may include silicon nitride, silicon oxynitride, and/or other suitable dielectric materials. For example, the work function metal layer may be a p-type or an n-type work function layer. The p-type work function layer comprises a metal with a sufficiently large effective work function, selected from but not restricted to the group of titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof. The n-type work function layer comprises a metal with sufficiently low effective work function, selected from but not restricted to the group of titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), or combinations thereof. The work function metal layer may include a plurality of layers and may be deposited by CVD, PVD, and/or other suitable process.

At operation 104, the method 100 (FIG. 1) etches trenches into the S/D regions 206 a-c adjacent the gate structures 208 b-c. Referring to FIG. 3, trenches 230 a, 230 b, and 230 c are formed into the S/D regions 206 a, 206 b, and 206 c, respectively, for growing epitaxial features therein in subsequent steps. In the present embodiment, the operation 104 includes multiple processes such as a dry etching process, an ion implantation process, a wet etching process, and/or a cleaning process. For example, a dry (anisotropic) etching process may be performed to form substantially U-shaped trenches into the substrate 202. Then, an ion, such as boron, is implanted into the active region 204 to change the crystalline structure of a portion of the active region. Subsequently, a wet (isotropic) etching process is performed to expand the U-shaped trenches. The etching rate in the ion-implanted portion of the active region 204 is higher than other portions. Consequently, the U-shaped trenches are turned into hexagonal shapes like the trench 230 b shown in FIG. 3. Then, a cleaning process may clean the trenches 230 a-c with DHF, HF, or other suitable solution. For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. For example, a wet etching process may comprise etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; TMAH solution; a solution containing hydrofluoric acid (HF), nitric acid (HNO₃), and/or acetic acid (CH₃COOH); or other suitable wet etchant. The etching processes are selective to the material of the substrate 202. In other words, the etching processes are tuned to remove the materials of the substrate 202 but not the isolation structures 212 a-b and the outer layers of the gate structures 220 a-c. As a result, the trenches 230 a and 230 c are not in hexagonal shape because one or more of their sidewalls are restricted by the respective isolation structures 212 a and 212 b.

Still referring to FIG. 3, the trench 230 a exposes a portion 232 a of a sidewall (or side surface) of the isolation structure 212 a. The portion 232 a becomes a sidewall of the trench 230 a. A sidewall 234 a of the trench 230 a is opposite to the sidewall 232 a with respect to a centerline of the trench 230 a. In the present embodiment, the sidewall 234 a is oriented in crystalline plane (1, 1, 1). Similarly, the trench 230 c exposes a portion 232 c of a sidewall of the isolation structure 212 b. The portion 232 c becomes a sidewall of the trench 230 c. A sidewall 234 c of the trench 230 c is opposite to the sidewall 232 c with respect to a centerline of the trench 230 c. In the present embodiment, the sidewall 234 c is also oriented in crystalline plane (1, 1, 1). Different from the trenches 230 a and 230 c, the trench 230 b is surrounded by semiconductor material(s) of the substrate 202, and has a hexagonal shape in this embodiment. The shapes of the trenches 230 a-c may be achieved by tuning parameters of the etching processes, such as etchants used, etching temperature, etching solution concentration, etching pressure, source power, radio frequency (RF) bias voltage, RF bias power, etchant flow rate, and other suitable parameters.

At operation 106, the method 100 (FIG. 1) epitaxially grows a first semiconductor layer 236, including features 236 a, 236 b, and 236 c, in the trenches 230 a-c. Referring to FIG. 4, the first semiconductor layer 236 a and 236 c only partially fill the trenches 230 a and 230 c respectively, while the first semiconductor layer 236 b completely fills the trench 230 b in the present embodiment. The different volumes in the first semiconductor layers 236 a-c are partially caused by the different materials on their sidewalls. Since the trench 230 b (FIG. 3) is surrounded by semiconductor material(s), epitaxial growth of the first semiconductor layer 236 b is promoted on all sides of the trench 230 b. In contrast, epitaxial growth of the first semiconductor layers 236 a and 236 c is restricted by the isolation structures 212 a and 212 b which comprise a dielectric material. As a result, the top surfaces (also side surfaces) 238 a and 238 c of the first semiconductor layers 236 a and 236 c, respectively, are slanted with respect to the top surface of the active region 204. In the present embodiment, the top surfaces 238 a and 238 c are oriented in crystalline plane (1, 1, 1). Further, top surface of the first semiconductor layer 236 b is oriented in crystalline plane (0, 0, 1) or an equivalent thereof. The semiconductor layers 236 a and 236 c may or may not be in direct contact with the isolation structures 212 a and 212 b, respectively, depending on the profile of the trenches 230 a and 230 c and the distance between the sidewalls of the isolation structures 212 a and 212 b and the centerline of the respective trenches 230 a and 230 c.

The first semiconductor layer 236 a-c may comprise silicon, silicon germanium (Si_(1-x)Ge_(x) or simply SiGe), or other suitable semiconductor material(s). In an embodiment, the first semiconductor layer 236 a-c is formed by one or more selective epitaxial growth (SEG) processes. In an embodiment, the SEG process is a low pressure chemical vapor deposition (LPCVD) process using a silicon-based precursor gas. Alternatively, the first semiconductor layer 236 a-c may be formed by cyclic deposition and etching (CDE) epitaxy, molecular beam epitaxy (MBE), or other suitable epitaxy techniques.

At operation 108, the method 100 (FIG. 1) dopes the first semiconductor layer 236 a-c with appropriate dopant(s). In an embodiment, the first semiconductor layer 236 a-c comprises silicon germanium (SiGe) for applying stress and improving charge carrier mobility for PMOS devices. To further this embodiment, the operation 108 dopes the silicon germanium layer 236 a-c with a p-type dopant, such as boron. The doping of the silicon germanium layer 236 a-c may be performed in-situ. In this case, operations 106 and 108 are performed simultaneously. For example, the epitaxial growth process may use boron-containing gases such as diborane (B₂H₆), other p-type dopant-containing gases, or a combination thereof to dope the silicon germanium layer 236 a-c with a p-type dopant in-situ. Alternatively, if the silicon germanium layer 236 a-c is not doped during the epitaxial growth process, it may be doped in a subsequent process (ex-situ), for example, by an ion implantation process, plasma immersion ion implantation (PIII) process, other process, or a combination thereof. In this case, the operation 108 is performed after the operation 106. An annealing process, such as a rapid thermal annealing and/or a laser thermal annealing, may be performed to activate dopants in the silicon germanium layer 236 a-c.

In another embodiment, the first semiconductor layer 236 a-c comprises silicon for applying stress and improving charge carrier mobility for NMOS devices. To further this embodiment, the operation 108 dopes the silicon layer 236 a-c with an n-type dopant, such as phosphorus, arsenic, or combinations thereof. Similar to the above discussion, the doping of the silicon layer 236 a-c may be performed in-situ or ex-situ.

At operation 110, the method 100 (FIG. 1) epitaxially grows a second semiconductor layer 240, including features 240 a, 240 b, and 240 c, over the first semiconductor layer 236 a-c. Referring to FIG. 5, the second semiconductor layer 240 a-c is disposed over top surfaces of the first semiconductor layer 236 a-c. In the present embodiment, the second semiconductor layer 240 a-c comprises silicon. In alternative embodiments, the second semiconductor layer 240 a-c comprises another elementary, compound, or alloy semiconductor material. In the present embodiment, the second semiconductor layer 240 a has a top surface (which is also a side surface) 242 a that is oriented in the crystalline plane (1, 1, 1), the second semiconductor layer 240 b has a top surface 242 b that is oriented in the crystalline plane (0, 0, 1) or an equivalent thereof, and the second semiconductor layer 240 c has a top surface (which is also a side surface) 242 c that is oriented in the crystalline plane (1, 1, 1). In embodiments, the second semiconductor layer 240 a-c may be epitaxially grown using SEG, MBE, CDE, or other suitable epitaxy techniques. For example, the second semiconductor layer 240 a-c may be epitaxially grown using a silicon-containing precursor gas, such as SiH₂Cl₂ (DCS).

It is noted that the first and second semiconductor layers 236 and 240 still only partially fill the trenches 230 a and 230 c because the epitaxial growth there is limited by the isolation structure 212 a-b. If S/D contact features were formed directly over the second semiconductor layer 240 a-c, the contact features would not land properly on the features 240 a and 240 c due to the slanted surfaces, which might lead to device defects (e.g., open circuits). Furthermore, the features 240 a and 240 c are thinner than the feature 240 b as measured along a direction that is normal to the respective top surfaces 242 a, 242 b, and 242 c. This is because the second semiconductor layer 240 (e.g., silicon) has a smaller growth rate in the crystalline plane (1, 1, 1) than in the crystalline plane (0, 0, 1). Therefore, the layers 240 a and 240 c may not have sufficient thickness for S/D contact formation. For example, S/D contact hole etching may completely penetrate the layers 240 a and 240 c, leading to increased S/D contact resistance. On the other hand, continuing the growth of the layers 240 a-c may cause overgrowth of the layer 240 b, which may lead to shorting of layer 240 b with nearby circuit features (not shown). In the present embodiment, the method 100 performs few subsequent processes to overcome the above issues.

At operation 112, the method 100 (FIG. 1) etches the second semiconductor layer 240 to change a crystalline facet orientation of at least a portion of the surfaces 242 a and 242 c. Referring to FIG. 6, the operation 112 produces new surfaces 244 a, 244 b, and 244 c on the second semiconductor layer 240 a, 240 b, and 240 c respectively. The crystalline facet orientation of the surface 244 b is about the same as the surface 242 b, though the layer 240 b may be reduced in its thickness along the Z direction which is normal to the top surface of the active region 204. The surfaces 244 a and 244 c have different crystalline facet orientation than the surfaces 242 a and 242 c, respectively. In the present embodiment, each of the surfaces 242 a and 242 c is in crystalline plane (1, 1, 1), and each of the surfaces 244 a and 244 c is in the crystalline plane (3, 1, 1) or its equivalent (1, 3, 1) and (1, 1, 3). In various embodiments, each of the surfaces 244 a and 244 c may be oriented in one of the crystalline planes of (3, 1, 1), (5, 1, 1), (7, 1, 1), (9, 1, 1), (1, 3, 1), (1, 5, 1), (1, 7, 1), (1, 9, 1), (1, 1, 3), (1, 1, 5), (1, 1, 7), and (1, 1, 9), which can also be expressed as {3, 1, 1}, {5, 1, 1}, {7, 1, 1}, and {9, 1, 1} for simplification. In the present embodiment, the operation 112 etches the second semiconductor layer 240 using a chemical having hydrogen chloride (HCl). Alternatively, the operation 112 may employ another chemical such as a hydride (e.g., HCl, HBr, HI, or HAt). The chemical etches the upper corner (see FIG. 5) of the layers 240 a and 240 c faster than it etches the lower body of the layers 240 a and 240 c, thereby forming the surfaces 244 a and 244 c. Furthermore, the chemical is tuned to selectively etch the second semiconductor layer 240 but not the gate structures 220 a-c and the isolation structures 212 a-b in the present embodiment.

At operation 114, the method 100 (FIG. 1) epitaxially grows a third semiconductor layer 246, including features 246 a, 246 b, and 246 c, over the second semiconductor layer 240 a-c (FIG. 7). The third semiconductor layer 246 may comprise silicon or other suitable semiconductor material(s). In various embodiments, the operation 114 may grow the third semiconductor layer 246 using SEG, MBE, CDE, or other epitaxy techniques. For example, the operation 114 may epitaxially grow the third semiconductor layer 246 using a silicon-containing precursor gas such as SiH₂Cl₂ (DCS) with 1% B₂H₆ gas.

Referring to FIG. 7, the features 246 a-c have multiple facets in their respective outer surfaces in the present embodiment. For example, the feature 246 a has a side surface 247 a and a top surface 248 a. The side surface 247 a is oriented in crystalline plane (1, 1, 1), and the top surface 248 a is oriented in crystalline plane (0, 0, 1) or an equivalent thereof, which is parallel to a top surface of the active region 204 in an embodiment. The side surface 247 a transitions to the top surface 248 a through one or more facets. The thickness of the layer 246 a increases from a lower part thereof (adjacent the isolation structure 212 a) to an upper part thereof (above the top surface of the active region 204).

Similarly, the feature 246 c has a side surface 247 c and a top surface 248 c. The side surface 247 c is oriented in crystalline plane (1, 1, 1), and the top surface 248 c is oriented in crystalline plane (0, 0, 1) or an equivalent thereof, which is parallel to the top surface of the active region 204 in an embodiment. The thickness of the layer 246 c increases from a lower part thereof (adjacent the isolation structure 212 b) to an upper part thereof (above the top surface of the active region 204). The feature 246 b provides a top surface 248 b oriented in crystalline plane (0, 0, 1) in the present embodiment.

The second and third semiconductor layers 240 and 246 collectively provide a desirably thick semiconductor layer for S/D contact landing. Particularly, the top surfaces 248 a and 248 c provide a flat or nearly flat surface for supporting S/D contacts to be formed thereon.

At operation 116, the method 100 (FIG. 1) dopes the third semiconductor layer 246 a-c with appropriate dopant(s). The third semiconductor layer 246 a-c may be doped in-situ (in which case, the operations 116 and 114 are performed simultaneously), or ex-situ (in which case, the operation 116 is performed after the operation 114), as discussed above with respect to the operation 108. In an exemplary embodiment, the third semiconductor layer 246 a-c comprises silicon and is in-situ doped with boron by using boron-containing gases such as diborane (B₂H₆) during the epitaxial growth process.

In the present embodiment, the dopant(s) applied to the third semiconductor layer 246 a-c is of the same type as the dopant(s) applied to the first semiconductor layer 236 a-c. For example, they are both p-type dopant(s), or are both n-type dopant(s). In a further embodiment, the first and third semiconductor layers 236 a-c and 246 a-c are doped with the same dopant, but the layer 246 a-c has a higher dopant concentration than the layer 236 a-c. One purpose of this configuration is to reduce contact resistance between the layer 246 a-c and S/D contact features to be formed thereon. In an example, the first semiconductor layer 236 a-c comprises silicon germanium doped with boron with a boron concentration ranging from 1E17 to 1E20 atoms/cm³, and the third semiconductor layer 246 a-c comprises silicon doped with boron with a boron concentration ranging from 1E20 to over 1E21 atoms/cm³. It is noted that the second semiconductor layer 240 a-c may or may not be intentionally doped. In some embodiments, the dopants in the layers 236 a-c and 246 a-c may diffuse into the second semiconductor layer 240 a-c, thereby doping the second semiconductor layer 240 a-c nonetheless. In some embodiments, the dopant concentration in the second semiconductor layer 240 a-c is lower than that of the third semiconductor layer 246 a-c, and is also lower than that of the first semiconductor layer 236 a-c at least at the boundary of the first and second semiconductor layers. In one example, the second semiconductor layer 240 a-c comprises silicon doped with boron with a boron concentration ranging from 1E19 to 1E20 atoms/cm³.

Still referring to FIG. 7, the method 100 has formed three epitaxial semiconductor layers 236 a-c, 240 a-c, and 246 a-c. Particularly, a three-layer epitaxial structure is formed in each of the S/D regions 206 a-c (FIG. 1). In the S/D region 206 a, the three-layer epitaxial structure includes the layers 236 a, 240 a, and 246 a bordering the isolation structure 212 a. Particularly, each of the layers 240 a and 246 a is in direct contact with the isolation structure 212 a. In the S/D region 206 b, the three-layer epitaxial structure includes the layers 236 b, 240 b, and 246 b surrounded by semiconductor material(s). In the S/D region 206 c, the three-layer epitaxial structure includes the layers 236 c, 240 c, and 246 c bordering the isolation structure 212 b. Particularly, each of the layers 240 c and 246 c is in direct contact with the isolation structure 212 b. In an embodiment, the first semiconductor layer 236 a-c has a thickness ranging from 20 to 40 nm, the second semiconductor layer 240 a-c has a thickness ranging from 2 to 10 nm, and the third semiconductor layer 246 a-c has a thickness ranging from 5 to 10 nm.

At operation 118, the method 100 (FIG. 1) forms an inter-layer dielectric (ILD) layer 250 over the substrate 202, the gate structures 220 a-c, the isolation structures 212 a-b, and the third semiconductor layer 246 a-c (FIG. 8). In an embodiment, the method 100 forms an etch stop layer (not shown) over the various structures before the forming of the ILD layer 250. Examples of materials that may be used to form the etch stop layer include silicon nitride, silicon oxide, silicon oxynitride, and/or other materials. The etch stop layer may be formed by PECVD process and/or other suitable deposition or oxidation processes. The ILD layer 250 may include materials such as tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/or other suitable dielectric materials. The ILD layer 250 may be deposited by a PECVD process, flowable CVD process, or other suitable deposition technique.

At operation 120, the method 100 (FIG. 1) forms conductive features 252 a-c in the ILD layer 250 and electrically contacting the third semiconductor layer 246 a-c, respectively. Referring to FIG. 9, the conductive feature 252 b is disposed on a flat surface of the third semiconductor layers 246 b, and the conductive features 252 a and 252 c are disposed on a relatively flat and thick part of the third semiconductor layers 246 a and 246 c, respectively. This advantageously provides good contact between the respective conductive feature and the semiconductor layer, and reduces the contact resistance thereof. The operation 120 may include a variety of processes including etching contact holes to expose the third semiconductor layer 246 a-c and depositing the conductive features 252 a-c in the contact holes. Each of the contact features 252 a-c may include multiple layers, such as a barrier/adhesion layer and a metal fill layer over the barrier/adhesion layer. For example, the barrier/adhesion layer may include titanium, titanium nitride, tantalum, tantalum nitride, a combination thereof, or other suitable materials. The barrier/adhesion layer may be formed by CVD, PVD, or other suitable processes. For example, the metal fill layer may include aluminum (Al), tungsten (W), cobalt (Co), copper (Cu), and/or other suitable materials. The metal fill layer may be formed by CVD, PVD, plating, and/or other suitable processes.

At operation 122, the method 100 (FIG. 1) performs other fabrication steps to the structure 200 in order to form a final IC product. For example, the method 100 may perform a gate replacement process. The gate replacement process replaces the gate dielectric layer 222 a-c and the gate electrode layer 224 a-c, which are originally silicon oxide and polysilicon in an embodiment, with a high-k gate dielectric layer and a metal gate electrode layer. The gate replacement process may be performed before or after the operation 120. For another example, the method 100 may form gate contacts over the gate structures 220 a-c. The gate contacts may be formed before, during, or after the operation 120. For yet another example, the method 100 may form an interconnect structure that connects the gate structures 220 a-c, the conductive features 252 a-c, and other parts of the device 200 (not shown). In a particular example, the interconnect structure may connect the gate structure 220 a with the conductive feature 252 a, in which case the gate structure 220 a functions as a local interconnect for electrically connecting the S/D feature (236 a/240 a/246 a) to a source, drain, or gate terminal of another transistor.

Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and the formation thereof. For example, embodiments of the present disclosure provide a three-layer epitaxial feature. The three-layer epitaxial feature provides good landing areas for S/D contact, which leads to reduced S/D contact resistance.

In one exemplary aspect, the present disclosure is directed to a method for semiconductor manufacturing. The method includes providing a semiconductor structure having an active region and an isolation structure adjacent to the active region, the active region having source and drain regions sandwiching a channel region for a transistor, the semiconductor structure further having a gate structure over the channel region. The method further includes etching a trench in one of the source and drain regions, wherein the trench exposes a portion of a sidewall of the isolation structure, epitaxially growing a first semiconductor layer in the trench, epitaxially growing a second semiconductor layer over the first semiconductor layer, changing a crystalline facet orientation of a portion of a top surface of the second semiconductor layer by an etching process, and epitaxially growing a third semiconductor layer over the second semiconductor layer after the changing of the crystalline facet orientation.

In another exemplary aspect, the present disclosure is directed to a method for making a semiconductor device. The method includes providing a semiconductor structure having an active region and an isolation structure adjacent to the active region, the active region having source and drain regions sandwiching a channel region for a transistor, the semiconductor structure further having a gate structure over the channel region. The method further includes etching a trench in one of the source and drain regions, wherein a first side surface of the trench is a portion of a sidewall of the isolation structure, and a second side surface of the trench is oriented in crystalline plane (1, 1, 1). The method further includes epitaxially growing a first semiconductor layer in the trench, and epitaxially growing a second semiconductor layer over the first semiconductor layer, wherein a top surface of the second semiconductor layer is oriented in crystalline plane (1, 1, 1). The method further includes etching the second semiconductor layer, thereby changing crystalline facet orientation of a portion of the top surface of the second semiconductor layer. The method further includes epitaxially growing a third semiconductor layer over the second semiconductor layer after the etching of the second semiconductor layer.

In another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a substrate having an active region, the active region having source and drain regions sandwiching a channel region. The semiconductor device further includes a gate structure over the channel region, an isolation structure at least partially embedded in the substrate, a first semiconductor layer embedded in a trench in one of the source and drain regions, a second semiconductor layer over the first semiconductor layer, and a third semiconductor layer over the second semiconductor layer. Each of the second and third semiconductor layers is in direct contact with the isolation structure. A first side surface of the second semiconductor layer is oriented in crystalline plane (1, 1, 1), and a second side surface of the second semiconductor layer is oriented in one of crystalline planes of {3, 1, 1}, {5, 1, 1}, {7, 1, 1}, and {9, 1, 1}.

The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device comprising: a substrate; a gate disposed on the substrate; a channel region in the substrate underlying the gate; an isolation structure disposed within the substrate; and a pair of first source/drain features disposed on opposite sides of the channel region, wherein a first source/drain feature of the pair includes: a first semiconductor layer having a top slanted surface extending along a same plane from the gate to the isolation structure; a second semiconductor layer having a topmost surface with a crystalline orientation other than (1, 1, 1); and a third semiconductor layer disposed on the second semiconductor layer.
 2. The semiconductor device of claim 1 wherein the second semiconductor layer further has a side surface that extends to the isolation feature.
 3. The semiconductor device of claim 2, wherein the second semiconductor layer and the third semiconductor layer physically contact the isolation feature and the second semiconductor layer is disposed between the first semiconductor layer and the isolation feature.
 4. The semiconductor device of claim 2, wherein each of the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer physically contacts the isolation feature.
 5. The semiconductor device of claim 2, wherein the side surface of the second semiconductor layer has a crystalline orientation of (1, 1, 1).
 6. A device comprising: a substrate; an isolation feature disposed within the substrate; a gate structure disposed on the substrate; a source/drain feature disposed within the substrate adjacent the gate structure, wherein the source/drain feature includes: a bottom semiconductor layer disposed on the substrate; a middle semiconductor layer disposed on the bottom semiconductor layer and extending to a sidewall of the isolation feature; and a top semiconductor layer disposed on the middle semiconductor layer and extending to the sidewall of the isolation feature; and an interlayer dielectric layer disposed on the gate structure and extend to the sidewall of the isolation feature.
 7. The device of claim 6, wherein the bottom semiconductor layer extends underneath the gate structure.
 8. The device of claim 6, wherein the bottom semiconductor layer physically contacts the isolation feature.
 9. The device of claim 6, wherein the middle semiconductor layer includes a topmost surface and a side surface that extends from the topmost surface, wherein the topmost surface and the side surface have different crystalline orientations.
 10. The device of claim 9, wherein the side surface of the middle semiconductor layer has an orientation of (1, 1, 1), and wherein the topmost surface of the middle semiconductor layer has an orientation from a group consisting of: {3, 1, 1}, {5, 1, 1}, {7, 1, 1}, and {9, 1, 1}.
 11. The device of claim 9, wherein the side surface of the middle semiconductor layer extends to the isolation feature.
 12. A semiconductor device, comprising: a substrate having an active region, the active region having source and drain regions sandwiching a channel region; a gate structure over the channel region; an isolation structure at least partially embedded in the substrate; a first semiconductor layer embedded in a trench in one of the source and drain regions; a second semiconductor layer over the first semiconductor layer; and a third semiconductor layer over the second semiconductor layer, wherein each of the second and third semiconductor layers is in direct contact with a sidewall of the isolation structure, wherein a first side surface of the second semiconductor layer is oriented in crystalline plane (1, 1, 1), and a second side surface of the second semiconductor layer is oriented in one of crystalline planes of {3, 1, 1}, {5, 1, 1}, {7, 1, 1}, and {9, 1, 1}; and an inter-layer dielectric (ILD) layer over the active region, the isolation structure, and the gate structure, wherein the ILD is in direct contact with the sidewall of the isolation structure.
 13. The semiconductor device of claim 12, wherein the first semiconductor layer comprises silicon germanium doped with a p-type dopant, and each of the second and third semiconductor layers comprises silicon doped with the p-type dopant.
 14. The semiconductor device of claim 12, further comprising a conductive feature embedded in the ILD layer and contacting the third semiconductor layer.
 15. The semiconductor device of claim 12, wherein the first side surface of the second semiconductor layer physically contacts the isolation structure.
 16. The semiconductor device of claim 12, wherein a top surface of the third semiconductor layer is parallel to a top surface of the active region.
 17. The semiconductor device of claim 12, wherein the third semiconductor layer extends above a bottom of the gate structure.
 18. The semiconductor device of claim 12, further comprising: another gate structure over the isolation structure.
 19. The semiconductor device of claim 12, wherein the first semiconductor layer includes: a first surface extending from the gate structure and into the substrate in a direction towards the channel region; a second surface extending from the first surface and into the substrate in a direction away from the channel region; a third surface extending from the second surface, wherein the third surface meets the second surface at an obtuse angle; and a fourth surface extending from the third surface to the second semiconductor layer.
 20. The semiconductor device of claim 19, wherein the third surface physically contacts the isolation structure. 